Java Interview Question 2

Java Fundamentals - Advanced

1. What is the difference between final, finally, and finalize?

• final: Keyword for constants, non-inheritable classes, non-overridable methods

• finally: Block that always executes in exception handling

• **finalize(): Method called by garbage collector before object destruction

2. Explain static keyword and its uses

• Static variables: Class-level variables, shared among all instances

• Static methods: Class-level methods, can be called without object

• Static block: Executes when class is loaded

• Static nested class: Nested class that doesn't require outer class instance

3. What is method overloading vs method overriding?

• Overloading: Same method name, different parameters (compile-time polymorphism)

• Overriding: Same method signature in child class (runtime polymorphism)

Memory Management & Performance

4. What are the different types of memory in JVM?

• Heap: Object storage

• Stack: Method calls and local variables

• Method Area: Class metadata

• PC Registers: Thread execution points

• Native Method Stack: Native method calls

5. How does garbage collection work in Java?

• Mark and Sweep: Marks reachable objects, sweeps unreachable ones

• Generational GC: Young Generation (Eden, S0, S1) and Old Generation

• Types: Serial, Parallel, CMS, G1 GC

6. What are memory leaks in Java and how to avoid them?

• Causes: Static collections, unclosed resources, listeners not removed

• Prevention: Use weak references, close resources in finally, avoid static collections

Collections Framework - Deep Dive

7. How does HashMap work internally?

• Uses array of buckets (Node<K,V>[] table)

• Hash function determines bucket index

• Handles collisions using linked list or tree (Java 8+)

• Load factor (0.75) triggers resizing

8. Difference between Comparable and Comparator

• Comparable: Natural ordering (compareTo() method)

• Comparator: Custom ordering (compare() method)

// Comparable
class Person implements Comparable<Person> {
    public int compareTo(Person other) {
        return this.name.compareTo(other.name);
    }
}

// Comparator
Comparator<Person> ageComparator = (p1, p2) -> p1.getAge() - p2.getAge();

9. What is ConcurrentHashMap and how is it different from HashMap?

• Thread-safe: Uses segment locking or CAS operations

• Better performance: than synchronized collections

• Doesn't allow null keys/values

Multithreading - Advanced

10. What is volatile keyword in Java?

• Ensures variable visibility across threads

• Prevents compiler reordering optimizations

• Doesn't provide atomicity (unlike synchronized)

11. Explain Executor framework

• Manages thread pools and execution

• Types: FixedThreadPool, CachedThreadPool, ScheduledThreadPool

ExecutorService executor = Executors.newFixedThreadPool(5);
executor.submit(() -> System.out.println("Task executed"));

12. What is deadlock and how to prevent it?

• Situation: Two or more threads waiting for each other indefinitely

• Prevention:

  • Lock ordering

  • Timeout on lock acquisition

  • Avoid nested locks

Java 8+ Features - Detailed

13. What are functional interfaces?

• Interfaces with exactly one abstract method

• Can have multiple default/static methods

• Examples: Runnable, Callable, Comparator

• New: Predicate, Function, Consumer, Supplier

14. Explain method references

• Shorthand for lambda expressions

// Instead of: list.forEach(s -> System.out.println(s));
list.forEach(System.out::println);

// Types:
// Static method: Class::staticMethod
// Instance method: instance::method
// Arbitrary object: Class::method
// Constructor: Class::new

15. What are the new Date/Time API features in Java 8?

• LocalDate: Date without time

• LocalTime: Time without date

• LocalDateTime: Date and time

• ZonedDateTime: Date, time with timezone

• Immutable and thread-safe

Design Patterns & Architecture

16. What is the Factory pattern?

• Creates objects without exposing creation logic

• Centralized object creation

public class AnimalFactory {
    public static Animal createAnimal(String type) {
        switch(type) {
            case "dog": return new Dog();
            case "cat": return new Cat();
            default: throw new IllegalArgumentException();
        }
    }
}

17. Explain the Builder pattern

• Constructs complex objects step by step

• Useful when object has many parameters

Person person = new Person.Builder()
    .name("John")
    .age(30)
    .address("123 Street")
    .build();

Spring Framework Basics

18. What is dependency injection?

• Objects receive their dependencies from external source rather than creating them

• Types: Constructor injection, Setter injection, Field injection

• Promotes loose coupling and testability

19. What are Spring Bean scopes?

• singleton: One instance per Spring container (default)

• prototype: New instance each time requested

• request: One instance per HTTP request

• session: One instance per HTTP session

• global-session: One instance per global HTTP session

Database & JDBC

20. What is JDBC and how does it work?

• Java Database Connectivity API

• Steps:

  1. Load driver: Class.forName("com.mysql.jdbc.Driver")

  2. Create connection: DriverManager.getConnection(url, user, pass)

  3. Create statement: connection.createStatement()

  4. Execute query: statement.executeQuery(sql)

  5. Process results: ResultSet

  6. Close resources

21. Difference between Statement, PreparedStatement, and CallableStatement

• Statement: Simple SQL, prone to SQL injection

• PreparedStatement: Precompiled, parameterized, prevents SQL injection

• CallableStatement: For stored procedures

Testing & Debugging

22. What is JUnit and how to write test cases?

• Unit testing framework

@Test
public void testAddition() {
    Calculator calc = new Calculator();
    assertEquals(5, calc.add(2, 3));
}

@BeforeEach // JUnit 5 (was @Before in JUnit 4)
public void setUp() {
    // Setup before each test
}

Security

23. What is the principle of least privilege in Java security?

• Code should run with only necessary permissions

• Implemented through SecurityManager and policy files

Microservices & Cloud

24. What are microservices and their advantages?

• Architecture: Small, independent, loosely coupled services

• Advantages:

  • Independent deployment

  • Technology diversity

  • Fault isolation

  • Scalability

System Design

25. How would you design a caching system in Java?

• Requirements: Thread-safe, eviction policy, memory management

• Implementation:

  • Use ConcurrentHashMap with expiration

  • LRU/LFU eviction policies

  • Consider distributed caching (Redis, Hazelcast)

  • Monitor cache hit/miss ratios

These questions cover advanced Java concepts, design patterns, framework knowledge, and system design principles that are crucial for senior Java developer positions and technical architecture roles.